Etymology[edit]
The English word "yak" is a loan originating from Tibetan:
གཡག་, Wylie: g.yag. In Tibetan, it refers only to the
male of the species, the female being called Tibetan:
འབྲི་, Wylie: 'bri, or nak. In English, as in most
other languages that have borrowed the word, "yak" is usually used for
both sexes.
Taxonomy[edit]
Yaks belong to the genus
BosBos and are therefore related to cattle (Bos
primigenius species).
Mitochondrial DNAMitochondrial DNA analyses to determine the
evolutionary history of yaks have been inconclusive.
The yak may have diverged from cattle at any point between one and
five million years ago, and there is some suggestion that it may be
more closely related to bison than to the other members of its
designated genus.[2] Apparent close fossil relatives of the yak, such
as
BosBos baikalensis, have been found in eastern Russia, suggesting a
possible route by which yak-like ancestors of the modern American
bison could have entered the Americas.[3]
The species was originally designated as
BosBos grunniens ("grunting ox")
by Linnaeus in 1766, but this name is now generally only considered to
refer to the domesticated form of the animal, with
BosBos mutus ("mute
ox") being the preferred name for the wild species. Although some
authors still consider the wild yak to be a subspecies,
BosBos grunniens
mutus, the ICZN made an official ruling in 2003[4] permitting the use
of the name
BosBos mutus for wild yaks, and this is now the more common
usage.[5][3][6]
Except where the wild yak is considered as a subspecies of Bos
grunniens, there are no recognised subspecies of yak.
Physical characteristics[edit]

A domestic yak at Yamdrok Lake.

Yaks are heavily built animals with a bulky frame, sturdy legs,
rounded cloven hooves, and extremely dense, long fur that hangs down
lower than the belly. While wild yaks are generally dark, blackish to
brown in colouration, domestic yaks can be quite variable in colour,
often having patches of rusty brown and cream. They have small ears
and a wide forehead, with smooth horns that are generally dark in
colour. In males (bulls), the horns sweep out from the sides of the
head, and then curve forward. They typically range from 48 to
99 cm (19 to 39 in) in length. The horns of females (cows)
are smaller, only 27 to 64 cm (11 to 25 in) in length, and
have a more upright shape. Both sexes have a short neck with a
pronounced hump over the shoulders, although this is larger and more
visible in males.[3] Males weigh 350 to 580 kg (770 to
1,280 lb), females weigh 225 to 255 kg (496 to 562 lb).
Wild yaks can be substantially heavier, bulls reaching weights of up
to 1,000 kilograms (2,200 lb).[7] Depending on the breed,
domestic yak males are 111–138 centimetres (44–54 in) high at
the withers, while females are 105–117 centimetres (41–46 in)
high at the withers.[8]
Both sexes have long shaggy hair with a dense woolly undercoat over
the chest, flanks, and thighs to insulate them from the cold.
Especially in bulls, this may form a long "skirt" that can reach the
ground. The tail is long and horselike rather than tufted like the
tails of cattle or bison. Domesticated yaks have a wide range of coat
colours, with some individuals being white, grey, brown, roan or
piebald. The udder in females and the scrotum in males are small and
hairy, as protection against the cold. Females have four teats.[3]
Yaks grunt and, unlike cattle, are not known to produce the
characteristic bovine lowing (mooing) sound, which inspired the
scientific name of the domestic yak variant,
BosBos grunniens (grunting
bull).
Nikolay PrzhevalskyNikolay Przhevalsky named the wild variant
BosBos mutus (silent
bull), believing that it did not make a sound at all.[9]
Physiology[edit]

Yak physiology is well adapted to high altitudes, having larger lungs
and heart than cattle found at lower altitudes, as well as greater
capacity for transporting oxygen through their blood[10] due to the
persistence of foetal haemoglobin throughout life.[11] Conversely,
yaks have trouble thriving at lower altitudes,[12] and are prone to
suffering from heat exhaustion above about 15 °C (59 °F).
Further adaptations to the cold include a thick layer of subcutaneous
fat, and an almost complete lack of functional sweat glands.[10]
Compared with domestic cattle, the rumen of yaks is unusually large,
relative to the omasum.[citation needed] This likely allows them to
consume greater quantities of low-quality food at a time, and to
ferment it longer so as to extract more nutrients.[10] Yak consume the
equivalent of 1% of their body weight daily while cattle require 3% to
maintain condition.[citation needed]
Odour[edit]
Contrary to popular belief, yak and their manure have little to no
detectable odour[13] when maintained appropriately in pastures or
paddocks with adequate access to forage and water. Yak's wool is
naturally odour resistant.[14]
Reproduction and life history[edit]

Ten-day-old yak.

Yaks mate in the summer, typically between July and September,
depending on the local environment. For the remainder of the year,
many bulls wander in small bachelor groups away from the large herds,
but, as the rut approaches, they become aggressive and regularly fight
among each other to establish dominance. In addition to non-violent
threat displays, bellowing, and scraping the ground with their horns,
bull yaks also compete more directly, repeatedly charging at each
other with heads lowered or sparring with their horns. Like bison, but
unlike cattle, males wallow in dry soil during the rut, often while
scent-marking with urine or dung.[3] Females enter oestrus up to four
times a year, and females are receptive only for a few hours in each
cycle.[15]
Gestation lasts between 257 and 270 days,[10] so that the young are
born between May and June, and results in the birth of a single calf.
The cow finds a secluded spot to give birth, but the calf is able to
walk within about ten minutes of birth, and the pair soon rejoin the
herd.[10] Females of both the wild and domestic forms typically give
birth only once every other year,[3] although more frequent births are
possible if the food supply is good.
Calves are weaned at one year and become independent shortly
thereafter. Wild calves are initially brown in colour, and only later
develop the darker adult hair. Females generally give birth for the
first time at three or four years of age,[16] and reach their peak
reproductive fitness at around six years. Yaks may live for more than
twenty years in domestication or captivity,[3] although it is likely
that this may be somewhat shorter in the wild.
Hybrid yak[edit]
In Nepal, Tibet and Mongolia, domestic cattle are crossbred with yaks.
This gives rise to the infertile male dzo མཛོ། as well as
fertile females known as མཛོ་མོ།dzomo or zhom, which may
be crossed again with cattle. The "Dwarf Lulu" breed, "the only Bos
primigenius taurus type of cattle in Nepal" has been tested for DNA
markers and found to be a mixture of both taurine and zebu types of
cattle (B. p. taurus and B. p. indicus) with yak.[17] According to the
International Veterinary Information Service, the low productivity of
second generation cattle-yak crosses makes them suitable only as meat
animals.[18]
Crosses between yaks and domestic cattle (
BosBos primigenius taurus) have
been recorded in Chinese literature for at least 2,000 years.[3]
Successful crosses have also been recorded between yak and American
bison,[18] gaur, and banteng, generally with similar results to those
produced with domestic cattle.[3]
Relationship with humans[edit]
Domesticated yaks have been kept for thousands of years, primarily for
their milk, fibre and meat, and as beasts of burden. Their dried
droppings are an important fuel, used all over Tibet, and are often
the only fuel available on the high treeless Tibetan Plateau. Yaks
transport goods across mountain passes for local farmers and traders
as well as for climbing and trekking expeditions. "Only one thing
makes it hard to use yaks for long journeys in barren regions. They
will not eat grain, which could be carried on the journey. They will
starve unless they can be brought to a place where there is
grass."[19] They also are used to draw ploughs.[20] Yak's milk is
often processed to a cheese called chhurpi in Tibetan and Nepali
languages, and byaslag in Mongolia. Butter made of yak's milk is an
ingredient of the butter tea that Tibetans consume in large
quantities,[21] and is also used in lamps and made into butter
sculptures used in religious festivities.[22]

Yak racing

Husbandry research[edit]
The Indian government established a dedicated centre for research into
yak husbandry, the ICAR-National Research Centre on Yak, in 1989. It
is located at Dirang, Arunachal Pradesh, and maintains a yak farm in
the Nyukmadung area at an altitude of 2750 m above MSL.[23]
Yak sports[edit]
In parts of Tibet and Karakorum, yak racing is a form of entertainment
at traditional festivals and is considered an important part of their
culture. More recently, sports involving domesticated yaks, such as
yak skiing or yak polo, are being marketed as tourist attractions in
Central Asian countries, including in Gilgit-Baltistan,
Pakistan.[citation needed]
Gallery[edit]